string builder vs string

Question

Possible Duplicates:
When to use StringBuilder?
string is immutable and stringbuilder is mutable

what is diffrent of string and string builder ? where do we use string builder ?

Answer 1

The main difference is that a StringBuilder is mutable (meaning that it can be modified), whereas a string is immutable (meaning that once it is constructed it cannot be modified).

This difference is important for example if you are trying to create a large string from lots of smaller strings. If you use a StringBuilder you append the strings without creating a new object, giving O(n) performance. If you use strings you create lots of intermediate strings which are immediately discarded, but all the extra copying means that it becomes an O(n²) operation.

Example code showing the usage of each for contructing a large string from many smaller strings:

String

string s = "";
for (int i = 0; i < 10000; ++i)
{
    s += "foo";
}

StringBuilder

StringBuilder sb = new StringBuilder();
for (int i = 0; i < 10000; ++i)
{
    sb.Append("foo");
}
string s = sb.ToString();

Answer 2

Aside from string being immutable and stringbuilder being mutable, you should use a string builder when you are going to be doing a lot of string concatenation, as that is a very expensive operation, especially when contained in a loop.